1. Field of the Invention
The present invention relates to an apparatus and method for determining the failure of an automatic transmission temperature sensor.
2. Discussion of the Prior Art
A transmission fluid temperature sensor (TFT) is used to detect a temperature of the transmission fluid. California Air Resources Board (CARB) has forced implementation of an "In-range TFT" diagnostic test to detect a TFT failure, because TFT is used as an entry condition for some transmission diagnostic monitors. CARB determined that TFT sensor may indirectly impact vehicle emissions and therefore requires this test.
Most prior art TFT diagnostic only detect open and short circuit faults. An improved test is described in U.S. Pat. No. 5,107,246 to Mogaki ('246 patent). The test described in the '246 patent includes measuring the temperature of the engine coolant and the transmission fluid. The test of the '246 patent then determines whether the transmission fluid temperature is within a predetermined range. But when the engine coolant temperature is above a particular value or below a second value, the transmission temperature sensor is not tested.
The test described in the '246 patent may still provide a false error detection if the engine coolant temperature sensor is working and the transmission fluid temperature sensor provides an erroneous signal within a predetermined range, or if the engine temperature sensor provides an erroneous signal within a predetermined range.
It would therefore be desirable to provide a test which determines more accurately proper operation of a transmission fluid temperature sensor.